1. Field of the Invention
The present invention relates to a method of manufacturing an optical disk.
2. Description of the Related Art
On an optical information recording medium, e.g., an optical disk, such as a compact disk (CD) or a video disk, information is recorded as the presence/absence (arrangement) of minute pits on tracks. In reproducing the information, an optical pickup, which traces on the tracks, detects a change in reflection light of a laser beam emitted on the pits as an electric signal.
Such optical disk have been manufactured as follows. First, a photoresist is coated on a glass master disk, and minute laser beam spots are then emitted on the master disk to deform those beam-hit regions of the photoresist by means of a cutting machine. The optically deformed regions are discretely formed in accordance with information signals. After the master disk has undergone a developing process, only the optically deformed regions are removed from the top of the master disk, leaving minute pits formed thereon. This master disk is subjected to nickel plating, then to electroforming to grow the nickel metal layer, thus forming a stamper. Using the stamper, information pits are transferred onto a transparent resin by an injection device, yielding a substrate. Metal, such as aluminum, is vapor-deposited on the substrate to form a reflection layer, and the resultant structure becomes an optical disk, such as CD.
According to the conventional information recording/reproducing system using such optical disks, the resolution of detectable information (sizes of pits or regions) is limited by the maximum spatial frequency of 2NA/.lambda. in light of the frequency characteristic, where NA is the number of apertures of the lens of the reproducing optical system and .lambda. is the wavelength of light used to detect recorded information. Since the practical, maximum spatial frequency of reproducible recorded information is approximately 1.0 to 1.5 times NA/.lambda., the high density of an optical disk is also limited.
Although various types of high-density optical disks have been developed, practical methods of manufacturing such high-density optical disks have not yet sufficiently been developed.